1. Field of the Invention
The present invention relates to a braking control system for controlling a braking force applied to each of four driven wheels of a four-wheel drive vehicle having a center differential gear, and more particularly to the control system which keeps a stable driving condition of the four-wheel drive vehicle when the vehicle goes downhill.
2. Description of the Related Arts
An ordinary passenger vehicle has a pair of wheels at each of its front and rear sides. Either the front wheels or the rear wheels of that vehicle are operatively connected with an engine to be driven directly thereby, while the rest of the wheels are not connected with the engine so as to be served as non-driven wheels. A vehicle having the driven wheels at its front side is called a front drive vehicle, while a vehicle having the driven wheels at its rear side is called a rear drive vehicle. And, a vehicle having the driven wheels at both of the front and rear sides is called a four-wheel drive (4WD) vehicle. As for a driving system of the four-wheel drive vehicle, various types of the system are known, such as a part time system, full time system, and the like. According to the full time system, all of the front and rear wheels are connected through a front differential gear, a rear differential gear, and a center differential gear.
Also, in order to prevent an acceleration slip from occurring in the case where an excessive driving force is applied when starting or accelerating the vehicle, an acceleration slip control system, i.e., a so-called traction control system is popular in the market, as disclosed in a Japanese Patent Laid-open Publication No.8-133054, for example.
According to the above-described part time system of the four-wheel drive vehicle, when the vehicle turns with its four wheels driven, its cornering maneuver is difficult due to a rotational difference between the front and rear wheels. This is called a phenomenon of tight corner braking. According to the full time system of the four-wheel drive vehicle, the driving force transmitted to the wheels through a transmission is effectively distributed to the front and rear wheels by the center differential gear, and the rotational difference between the front and rear wheels is compensated, so that a smooth cornering maneuver can be ensured. However, the center differential gear will cause another problem. That is, if one of the front and rear wheels slips to rotate freely, the driving force will not be transmitted to the rest of the wheels at all. In order to avoid this, a center differential locking mechanism for locking the center differential gear manually has been introduced.
According to the full time system of the four-wheel drive vehicle, however, if the center differential gear is locked by the center differential locking mechanism, the phenomenon of tight corner braking will be caused, as described before. As a result, the vehicle's cornering maneuver will be difficult. In this case, therefore, the vehicle's driver must reduce the vehicle speed slow enough to turn the vehicle properly. As a countermeasure against this, simply removing the center differential locking mechanism will need another countermeasure, when one of the wheels will slip to rotate freely, as described before. The traction control system may be used as that countermeasure, so that the vehicle can be pulled out from a mud, for example. However, when the vehicle travels on a rough and downhill road with an engine brake operated, for example, if one of the wheels slips to rotate freely, the engine torque to be used for braking the wheels will not be transmitted to the wheels which contact the ground in the vehicle's path (hereinafter, simply referred to as contact wheel), but to the freely rotating wheel (hereinafter, simply referred to as non-contact wheels). According to the present application, the non-contact wheel is not limited to mean the freely rotating wheel, but means such a wheel that is substantially unable to transmit a load of the vehicle to the ground. Thus, the engine torque will be transmitted to the non-contact wheel, to rotate the same in the reverse direction. When the engine brake is operated on the downhill road, therefore, a certain countermeasure is needed. As a countermeasure in that case, a braking force corresponding to the engine brake may be applied to the front wheels, considering a load shift of the vehicle, which is caused when the vehicle goes downhill with the engine brake operated. As another countermeasure in that case, a braking force may be applied to the non-contact wheel, with the engine brake applied to the rest of the wheels properly.
In the case where those countermeasures are employed, the non-contact wheel is rotated in the reverse direction, as described above, so that if the wheel speed of the wheel rotating in the vehicle's moving direction is of a positive value, the wheel speed of the non-contact wheel will be of a negative value. However, an ordinary wheel speed sensor used for detecting the wheel speed can not identify its rotating direction. Since the sensor can not distinguish between the normal rotation and the reverse rotation, its output signal indicates the positive value, even if it is output when the wheel is rotating in the reverse direction. Therefore, the non-contact wheel is erroneously determined to be contacting the ground. As a result, the non-contact wheel or the front wheels may not be applied with a desired braking force. As a countermeasure to this, it is possible to provide a device for enabling the sensor to identify the rotational direction, but the device or its control will be complicated.